Chapter 9 Nested Loops and TwoDimensional Arrays

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Chapter 9Nested Loops and Two-Dimensional Arrays

• Lecture Slides to Accompany
• An Introduction to Computer Science Using Java
  (2nd Edition)
• by
• S.N. Kamin, D. Mickunas, E. Reingold

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Chapter Preview

• In this chapter we will
• show how nested loops are useful
• introduce two-dimensional arrays
• describe the use of two-dimensional arrays to
  represent grids of information
• show how computer graphics are generated using
  pixels

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Nested for Loops

• Nested loops frequently used to process
  two-dimensional arrays
• Often body of inner loop is where the main
  computation is done
• Example
• for (i 0 i lt m I)
• before inner loop
• for (j 0 j lt n j)
• body of inner loop
• after inner loop

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Dependent for Loops

• Sometimes the extent of the inner nested loop
  will depend on the index value of the outer loop
• Example
• for (i 0 i lt 3 i)
• out.print(i i j )
• for (j 0 j lt i j)
• out.print( j)

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Nested Loop Contained in Other Statements

• for (int i 1 i lt 10 i)
• if (i 2 0) // i even
• for (int j 1 j lt i/2 j)
• out.print()
• else // i odd
• for (int k 1 k lt 5 i/2 k)
• out.print()
• out.println()

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Output

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Two-Dimensional Arrays

• Declaration similar to one dimensional arrays
• Need to specify both the number of rows and
  columns during allocation
• Example
• final int
• COLS 6,
• ROWS 5
• double
• energyTable new doubleROWSCOLS

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Computing Row Totals

• double yearTotals new doubleROWS
• for (y 0 y lt ROWS y)
• // compute total for year y
• yearTotalsy 0.0
• for (s 0 s lt COLS s)
• yearTotalsy
• yearTotalsy energyTotalys

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Populating energyTable

• int y, s
• // reads 30 numbers needed to fill
• // energyTable one row at a time
• for (y 0 y lt ROWS y)
• for (s 0 s lt COLS s)
• energyTableys in.readDouble()

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Initializing Two-Dimensional Arrays

• double energyTable
• 18.9, 19.4, 34.2, 3.9, 5.7, 0.3,
• 19.1, 19.3, 33.6, 3.0, 6.2, 0.2,
• 18.8, 19.6, 32.9, 3.1, 6.6, 0.2,
• 18.9, 20.3, 33.5, 2.8, 6.7, 0.2,
• 19.6, 20.8, 33.8, 3.1, 6.5, 0.2

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Arrays of Arrays

• When we write
• energyTable new doubleROWSCOLS
• This is shorthand for
• energyTable new doubleROWS
• for (int i 0 i lt ROWS i)
• energyTableI new doubleCOLS

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Computer Graphics

• Computer graphics is the study of methods of
  representing and manipulating images
• A two-dimensional array can be used to represent
  the image to be displayed
• This array is called a frame buffer it has one
  entry for each pixel giving its color
• The number of pixels in the frame buffer is the
  resolution the display device

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Bresenhams Line Drawing Algorithm

• Draw (x0, y0) and compute p0 2?y - ?x
• Repeat for values of i from 1 to ?x 1
• Calculate xi1 xi 1
• Calculate yi1 yi 1, if pi gt 0
• yi ,
  otherwise
• Draw a pixel at (xi1, yi1)
• Compute pi pi 2?y - 2?x(yi1 - yi)

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Two-Dimensional Arrays and length

• A.length is number of rows in two-dimensional
  array A
• Ai.length is number of columns in row i from
  two-dimensional array A
• In Java rows can be of different lengths
• Example
• int A new int5
• for (int i 0 i lt 5 i)
• Ai new inti 1